Predicting biodiversity: a practical alternative?

It is not necessary to restate the obvious: that species are being lost because of our seemingly careless attitude toward maintaining biodiversity or in general, the health of our environment. Too many scientific papers document the effects of our harmful influence on the environment at regional and global scales, but what are the alternatives?

Indeed, does Michael Rosenzweig offer us a convincing alternative?

Rosenzweig argues that area is an important parameter that can predict biodiversity; in most cases, a logarithmic relationship between species and increasing area yields a positive slope. Thus, a percent reduction in area would imply a percent loss of species.

These species-area relationships (SPARs) yield similar trends when regional diversity is used to predict local diversity—echo patterns. However, archipelagoes yield an asymptotic curve with local diversity reaching a plateau and not a linear increase in species. Rosenzweig argues that this might be in part because the islands are sampled as one province rather than individual regions. If this is not a question of sampling, from a solely speculative angle, might this suggest the involvement of a dynamic phase (expansion of a species range) that regulates abundance between archipelagoes? Still, these patterns remain phenomena because the mechanisms underlying the relationship between the variables are unknown.

What we do know is that a loss in area affects species in a plethora of ways—for example, habitat loss and redistribution and/or reduction of range size. But how do small range sizes imply a reduced rate of speciation? This nagging question (a statement by Rosenzweig) teases my mind to only generate further questions: what does range size have to do with the rate of speciation? And further, is Rosenzweig using range size as an assumption to hold the species-area relationship that drives the concept of reconciliation ecology? Let’s suppose that large ranges have a lower rate of speciation. Doesn’t this assumption by itself undermine the species-area relationship? Because larger areas reflect a greater number of species and since “species are nurseries for other species”, in theory, a large range would have a higher rate of speciation. It seems Rosenzweig doesn’t offer a strong theoretical or observational correlation between range size and rate of speciation.

However, we concur with Rosenzweig that some standard conservation efforts are not effective and we all know Dr. Stoner’s perspective on the issue. Our modified ecological landscape presents us with the perception that humans exist amid these island-like patches of biodiversity or the idea that nature somehow exists decoupled from human habitation. Here, the concept of reconciliation ecology informed by the basis of SPARs finds its footing. The modified landscape (for e.g., from loss of area) can be used to maintain biodiversity levels. The string of examples (Rosenzweig 2003, p. 201-203) provide a convincing case for the fresh concept. But we could have ten alternatives and still be faced with the same problem because this in some sense requires a shift in our thinking, right?

Another tool for assessing changes in species distribution based on future climatic scenarios are climate envelopes. Like SPARs, climate envelopes do not address specific threats that affect the behavior and physiology of a species or human-related causes such as pollution and urban development on the redistribution of species. The Ibanez et al. review work is effective if area and climate envelopes are not used as sole indicators of change in diversity since they do not address in general, complex interactions. However, the review falls short of suggesting a particular predictive path or a pattern among the suggested approaches for estimating diversity. Rather, the focus is drawn to a series of questions that provoke a rethinking of current approaches.

Perhaps more interesting is the Butler et al. (2007) work on predicting mean risk score of farmland birds in the UK based on six components of agricultural intensification. The model assumes that the agricultural practices will have an effect on “diet, foraging habitat and nesting success”. In my opinion, it also assumes a uniform effect of agricultural intensification but in reality one of these six practices may affect a habitat more severely than another. Risk scores are related to the conservation status of the species in the UK; lower risk scores reflect a species that has a broad niche and the least severe conservation status. In addition, species with high risk scores also have a lower population growth rate.

The effects of using a genetically-modified herbicide-tolerant (GMHT) cropping system were examined; this method predicts a limited effect on farmland bird index (FBI) while reducing “above-ground invertebrates and weeds”. However, even under a 2020 scenario, current management schemes and the GMHT system will produce a positive growth rate only in a limited number of farmland species.

In some strange way, the predictive/theoretical aspect of biology seems more concrete given some failures of past efforts or the seeming lack of evaluation of effectiveness of current strategies.

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