Introduction

Tropical biologists have had a hard time predicting the future of tropical forests. Such predictions usually focus on worst-case scenarios such as the Goodland and Irwin [1] mixed metaphor: “Amazon jungle: from green hell to red desert.” Similarly, the first effort for an integrated focus on tropical ecosystem studies had in its title the assumption of “fragile ecosystems” [2]. In fact, the list of myths associated with tropical forests is long and continues to cause disagreements (Table 1). By myth I mean generalizations encompassing all tropical forests that are unproved or false, or which could apply to particular situations but cannot be generalized to all tropical forests. More recently, the predictions of species extinctions have become controversial, particularly in light of early expectations that have proven to be overestimates of extinction rates based on overestimates of deforestation rates and population growth [7891011121314151617–18].

Table 1.

Popular tropical forestry myths. See [3] for a longer list.

Nevertheless, in spite of efforts to reduce the level of uncertainty, the debates about tropical forests rage on and are bound to become more contentious as we try to anticipate the effects of global change on tropical forests. Moreover, the study of the resiliency and adaptive capacities of tropical forests remains in its infancy. For example, a volume edited by A. Markham [19] contains an excellent analysis of the potential negative effects of climate change on tropical forests, but none of the 23 articles considered the capacity of tropical forests to adjust to climate change through the emergence of communities of plants and animals with novel species combinations that maintain familiar ecosystem functions and services [20]. Here I elaborate on this possibility, but first address the reasons why myths about tropical forests develop. Further studies focused on how tropical forests function in the context of anthropogenic effects are required to overcome some of these misconceptions.

Why So Many Myths?

A shortage of scientific knowledge limits the ability to predict the future of tropical forests. In part this is due to the low level of scientific activity in the most complex and extensive of all forest regions of the world [21]. Disagreement among scientists is partially due to the challenges faced by scientific activity in the tropics. These include: complex ecosystems and complex environmental situations, shortage of empirical information and understanding, and unrestrained generalizations across temporal and spatial scales, which many times also involve crossing disciplines without proper precautions. An example of the danger of crossing scales and disciplines would be the extrapolation of laboratory leaf- or plant-level physiological work to whole ecosystem-level ecological processes. At the ecosystem level of functioning, leaves and individual plants are subject to nutrient, light, and water limitations and conditions that are difficult to reproduce in the laboratory, which limits the validity of extrapolating results from the laboratory to the whole system. Also, there are problems of scale when extrapolating from small-scale, short-term studies to large-scale, long-term phenomena (Table 2). Moreover, much of the ecological research since the publication of Fragile Ecosystems [2] focused on mature native forests, and less attention was paid to secondary forests responding to anthropogenic disturbances, and even less to forests on degraded lands. Yet today the area of secondary forests is greater than the area of mature forests and we are entering an era of novel forests [27], which developed on degraded lands with combinations of species that are different from those of historic native forests [28].

We Need More Attention to Mechanisms of Persistence

One key shortcoming among scientific debates about the tropics is the low level of attention given to persistence mechanisms that continually undermine predictions of worst-case scenarios, particularly of extinction rates related to climate change [29]. For example, tropical forests are often considered ecologically fragile, yet these same forests also have been known to overcome large-scale intensive disturbances and recover after deforestation [4]. Tropical forests, though vulnerable to human activity, particularly deforestation and urbanization, are also resilient and capable of adjusting to environmental change through self-organization and other mechanisms of persistence, which allow populations and communities to overcome disturbances. The following examples of mechanisms of persistence are pantropical.

The tropical ecology literature is now expanding in all these areas of research (see review [30]) and collectively they suggest the emergence of new paradigms of tropical forest persistence in the Anthropocene [27], also known as the Homogeocene.

Table 2.

Problematic fallacies about tropical forests.

The Anthropocene is Again Dividing Scientific Opinion

There is no question that humans are now a dominant factor influencing the world's environment. Figure 1 illustrates the level of anthropogenic effects on critical global geochemical cycles. These altered geochemical cycles in turn affect ecosystem functioning and global climate. Two opposite points of view have developed regarding future scenarios for global biota in light of human effects on climate and the environment in general. The pessimistic scenario is summarized in Table 3 and is well documented [21, 19] This scenario anticipates rampant species invasions, catastrophic levels of species extinctions, homogenization of the biota, and disruptions of ecosystem services. The alternative scenario is that the biota will adapt and adjust to the Anthropocene and will do so by remixing species into novel ecosystems with familiar functions and ecological services.

Fig. 1.

Level of anthropogenic effects on critical global geochemical cycles. Data are from Sterner and Elser [31].

Here I present the alternative scenario using the island of Puerto Rico as a case study [36]. Although this discussion mainly focuses on the emergence of novel forests from an ecological perspective, I briefly review the socioeconomic and political forces, explored thoroughly elsewhere [37], that allow for this emergence. The influence of socioeconomic phenomena on forest cover has often been discussed within the context of the Forest Transition model, which describes the tendency for forest cover to decrease due to colonization and population growth, but then subsequently rebound as societies undergo economic development, industrialization and urbanization [38]. Therefore, under the right circumstances, economic development can have positive feedback relationships with forest cover, at regional scales. The Forest Transition model has global applicability [29, 40], having been documented in tropical countries as varied as Costa Rica, Cuba, Dominican Republic, The Gambia, Morocco, and Rwanda, and suggesting that the events in Puerto Rico are not unique. We now know that understanding the fate of natural systems requires incorporation of the influence of human systems. Moreover, the combination of diverse disciplines to address these questions challenges the mythology of individual disciplines and broadens such understanding.

Table 3.

The Anthropocene: the era of human domination of the biosphere when the world's biota will be homogenized. Each “Predicted Scenario” is drawn from McKinney and Lockwood [32]. Comments are by the author.

Puerto Rican Case Study

In Puerto Rico, deforestation, agricultural development, and land degradation occurred over several centuries after the 17^th century [41]. By the mid-20^th century 93 percent of all forest cover and 99 percent of primary forest had been converted to agriculture, mostly for the cultivation of sugar cane, tobacco, and coffee. In the 1990s, the landscape was extremely fragmented, with about six miles of road per square mile of land, splitting the landscape into 20,000 forest fragments, such that the predominant patch size was 1 ha or less [42]. How has the biota responded to such an extreme record of human manipulation?

Surprisingly, documented extinction rates of both plants and animals have been well below those expected from species-area analysis [43]. Moreover, following land abandonment due to economic development, industrialization and urbanization, forest cover has increased from less than 5 percent in mid-20th century to over 50 percent today [44]. In the process, forest patch sizes increased in area while decreasing in numbers. Today, forested watersheds in private lands supply water, support wildlife, and deliver other ecological services to a highly urbanized human population. The Puerto Rican example does not conform to the worst-case scenarios of the Anthropocene, and one could ask if there are lessons from this case study that might apply to other tropical locations. The unique species composition and species diversity of the emerging forests of Puerto Rico, termed novel forests [28] and described below, may represent the future of tropical forests [45].

Puerto Rico's Novel Forests

Results from island-wide forest inventories in Puerto Rico have demonstrated that 75 percent of forests on the island are novel forests [44, 46, 47] and have maintained a steady increase in area converted from abandoned agricultural lands [48]. Their species composition possesses several attributes that distinguish them from undisturbed forests [46, 33, 48]. Canopy species are dominated mostly by introduced species associated with past human activity, while forest understories remain dominated by native species [4950–51], [52]. Additionally, the number of tree species per hectare is higher in these forests than in mature native forests. Between 1982 and 2002, the abundance and importance value (measured by the relative tree density and basal area of each species) of individual species changed, but overall the importance value of introduced and naturalized tree species increased. For example, one species, the African tulip tree (Spathodea campanulata), has become the most common tree species on the island.

Over time, the resurgent forest canopy has diversified into a mix of introduced and native species that have survived hurricanes. Survival of hurricanes is a key element in the naturalization of introduced species, as those that cannot cope with natural disturbances would not prevail in the future forests of the island. Native species, including primary, secondary, and endemic forest species, have persisted alongside introduced species [4950–51], [52], and species homogenization has not been observed at the local or landscape scale [50]. These novel forests have structural attributes such as basal area, canopy height, tree density, and wood volume that are similar to those of native forests. Moreover, their net primary productivity, biomass accumulation, and nutrient fluxes are greater than those of the native forests they replaced [53, 54]. Animal communities in these novel forests consist of mostly native (e.g., birds, amphibians, and reptiles) and combinations of native and introduced species (e.g., earthworms) [55, 56], [34].

Analysis of the island's tree flora has shown that introduced and naturalized species now comprise 23 percent of the flora, and that these species are becoming permanent components of the island's forests [57]. The novel species composition of these forests concurs with observations elsewhere in the world, where the species composition of emerging forests on degraded lands includes combinations of species not recorded before in those locations [28, 58, 59].

Table 4 summarizes some of the characteristics of Puerto Rico's novel forests. The presence and natural development of these forests are relevant to the establishment of vegetation on degraded sites, the colonization of sites where environmental conditions are novel, adaptation to global change, succession after land use change, and aboveground carbon storage. For example, many native primary and secondary forest species are unable to grow on degraded lands, allowing introduced species to prevail [63, 64]. Once forest conditions are restored by the growth of introduced species and the canopy closes, those native species that could not grow on the degraded site establish viable populations in the understory and eventually reach the canopy, thus adding species diversity to the novel forest [63].

Table 4.

Some generalizations about the structural and functional attributes of novel forests in Puerto Rico. The citations are illustrative of many additional examples.

Predictions

From the experience in Puerto Rico and elsewhere [59], it appears that new combinations of introduced and native tree species will persist where humans dominate the environment. It is unlikely that the pre-existing native species combinations will replace the current novel species assemblages because the naturalized introduced species are capable of regeneration under the prevailing environmental conditions of the island, i.e., both natural and anthropogenic disturbance regimes. When anticipating climate change effects, it is important to consider the synergy between any climate change parameter and other anthropogenic disturbances such as urbanization or land degradation. In other words, climate change effects should not be considered in isolation, because in the real world they interact with other anthropogenic disturbances. Hence I focus here on global change, which includes both types of disturbances. Continuous global change favors the persistence of introduced species and formation of novel communities. One could use observations from the Puerto Rican case study to make the following predictions for the future of tropical forests in a world dominated by anthropogenic activity and experiencing global change:

These predictions contrast those of worst-case scenarios for the tropics, but they merit research attention given that the experience in places like Puerto Rico shows that if natural systems are allowed space, they expand and adjust to emerging conditions rather than self-destruct. The consequences of either scenario are of importance to the public, governments, and conservationists in general. Each scenario has different costs and requires different levels of effort to address them. For example, restoring natural vegetation on small islands through the eradication of introduced animals can be costly with unpredictable effects and controversy [65], [6667–68], while natural processes at no cost to the economy powered the reforestation process that led to increasing the forest area in Puerto Rico from 6 percent to 57 percent. We face a conundrum as articulated by Botkin [69]: “One can either preserve a ‘natural’ condition, or one can preserve natural processes, but not both.” If in the future we need to reverse natural trends to accomplish our conservation goals, the cost will be enormous and probably unattainable. If however, the natural responses of forests to anthropogenic effects are compatible with our goals of sustainable development, the costs associated with conservation might be within our reach. Only in time will we find out.

Acknowledgments

This article is based on a keynote address at the International Society of Tropical Foresters Annual Conference, “Tropical Forests Under a Changing Climate: Linking Impacts, Mitigation, and Adaptation,” February 11–13, 2010, Yale University. This study was done in cooperation with the University of Puerto Rico. I thank Mildred Alayón for support with the editing and three anonymous reviewers for suggestions that improved the manuscript.